The research and development of micro-optical circuits are being extensively made in optical communication systems using waveform multiplexing. When an optical device using an optical waveguide made of materials having a large refractive index difference is achieved, a lower-cost, lower-power-consumption optical communication apparatus than the conventional apparatuses can be implemented. Also, the developed optical device element technologies are applicable to optical interconnections on an LSI chip or between chips, and expected as means for avoiding various technical difficulties resulting from the increase in transmission capacitance of the conventional electrical interconnections.
Representative examples of the optical waveguide using materials having a large refractive index difference are a ridge waveguide using Si as a waveguide core on an SOI (Silicon on Insulator) substrate as described in Japanese Patent Laid-Open No. 7-168146, a rib waveguide, a strip waveguide, and a photonic crystal line defect waveguide. In addition, the research and development are being extensively made on an optical waveguide using a compound semiconductor such as GaAs as described in Japanese Patent Laid-Open No. 10-221554.
The optical waveguides as described above have functions of devices in some cases, and are often used as optical interconnecting elements connecting functional elements such as light emitting elements, light receiving elements, light branching elements, light coupling elements, optical demultiplexing elements, optical multiplexing elements, light modulating elements, optical switching elements, optical memory elements, and optical buffer elements integrated on a micro-optical circuit. From this viewpoint, smooth conversion and low-loss connection of an electromagnetic field mode propagating in a waveguide in a portion connecting a waveguide and various functional elements or in a portion connecting waveguides different in material or size are extremely important in every optical device having an optical waveguide.
From this point of view, the connection of two types of strip waveguides (including an optical fiber connecting spot size converter) (e.g., T. Shoji et al., Electronics Letters, vol. 38 (2002). pp. 1669-1670, M. Yanagisawa et al., IEEE Photonics Technology Letters, vol. 4 (1993) pp. 433-435, and Japanese Patent Laid-Open Nos. 7-168146, 10-221554, 2006-146127, 2005-326876, and 2005-275064), and the connection of a strip waveguide and photonic crystal line defect waveguide (see, e.g., A. Gomyo et al., IEICE Trans. Electron., Vol. E87 (2004) pp. 328-335, J. Ushida et al. Phys. Rev. B68, (2003) pp. 155115-1 to 7, and International Application WO2004/081627) have conventionally been researched and developed, and various connecting structures have been examined and proposed.